A magnetic disk may be employed for the storage of video pictures, for example, one video field per recorded track. It is sometimes desirable to erase a single track, that is, a single field of a recorded video picture, without disturbing the fields recorded on adjacent tracks. It is also important that most of the picture content be removed so that a new picture can be recorded on the erased track without interference from any recorded remnants of the old picture. Typically, the disk is rotated while the erasure takes place. An alternating current erasing signal is applied to a magnetic head that generates an erasing field. The head is closely positioned in relation to the rotating magnetic disk so that the erasing field interfaces with the track and erases the resident signal.
Particularly with a video signal having many high frequencies, it is desirable to use an erasing signal with even higher frequencies (for example, an erasing frequency exceeding 20 mHz for erasing recorded video frequencies). The frequency response of the electrical circuit processing the erasing signal ordinarily, however, bars the use of such high frequencies. One way of increasing the erasing frequency without exceeding the circuit's capability is to operate the disk at a lower rotational velocity than its normal record or playback velocity (see the background discussion in U.S. Pat. No. 3,723,668). This has the effect of impressing a relatively larger number of frequency alternations on a given track for a relatively lower frequency erasing current. Despite this technique, however, it is still difficult to erase all of the video picture content with obtainable erasing frequencies.